The invention is directed to processes and apparatuses for crystallization, in particular for continuous crystallization.
Although continuous crystallization provides a number of advantages, a significant fraction of the industrial sized crystallization processes are still carried out in a batch-wise fashion. One example of an industrial sized continuous crystallization process is the MSMPR (Mixed Slurry Mixed Product Removal) process. However, the problem with MSMPR is that it is difficult to generate particles with a narrow size distribution and to control the morphology. The same holds when a cascade of crystallizers is used. Moreover, the advantages of a real continuous process are not fully attained with these known processes. These advantages include inter alia a constant product quality, a smaller reactor volume and more flexibility. Especially for precipitation, which term refers to the relatively fast crystallization processes of slightly soluble salts, it is in practice very hard to keep the product quality constant during scale up of a crystallization process. This is due to the comparable timescale of mixing and reaction, which may however scale up differently. Consequently, scale up of a precipitation process in continuous stirred tank reactors often leads unwanted changes in size and/or shape and/or polymorph. Polymorphs are substances that have the same chemical formula, but a different crystal structure. This difference in crystal structure induces changes in the physical and chemical properties of the materials, such as e.g. solubility density, stability, etc. Clearly the control of such properties is of upmost importance for commercial crystalline products like pharmaceuticals and fertilizers. Calcite, aragonite and vaterite are for example polymorphs of calcium carbonate (CaCO3).
DE-A-39 20 999 describes a process for continuous crystallization of nitroguanidin. Crystallization is initialized by cooling of a reactor in a tube and shell configuration. A spiral tube is used in order to quickly cool the solution. It is claimed that in this way only primary nucleation occurs and that secondary nucleation is prevented. The growth of the thus-generated primary nuclei takes place in cascade of mixed tank reactors, placed down stream of the spiral reactor.